The present invention relates to a magnetron cathode assembly and, more particularly, to a magnetron cathode assembly wherein a resistance to vibration of a filament thereof is improved.
In a conventional magnetron cathode assembly, the following problem is presented. A center lead and a side lead are simply fitted in a spacer for preventing them from vibration. For example, when the center lead starts vibration, the spacer is pivoted about the side lead. In other words, the antiresonance properties of the spacer have directivity, thus failing to provide sufficient mechanical strength. This drawback will be described in detail with reference to FIGS. 1A and 1B which structure is of the type described in U.S. Pat. No. 4,558,250, isued on Dec. 10, 1985 to M. Tsuzurabara.
FIG. 1A is a longitudinal sectional view of a conventional magnetron cathode assembly, and FIG. 1B is a cross-sectional view thereof taken along the line I-I' of FIG. 1. Referring to FIG. 1A, a filament 1 is arranged along the axis of the magnetron to emit thermoelectrons and clamped between an upper end shield 2 and a lower end shield 3 which prevent the filament 1 from removal. A center lead 4 and a side lead 5 are connected to the upper and lower end shields 2 and 3, respectively. Terminals 6 and a ceramic stem 7 are welded by silver-copper brazing to the lower ends of the center and side leads 4 and 5, respectively. The center and side leads 4 and 5 are respectively fitted in through holes formed in an insulating spacer 8. The insulating spacer 8 serves to prevent the leads 4 and 5 from being disconnected due to external vibration. A sleeve 9 is welded on one (in the example, the sleeve 9 is welded on the side lead 5) of the center and side leads 4 and 5 to position the spacer 8.
Although the antiresonance properties are provided by the spacer, resonance occurs since the center and side leads have different lengths and different resonance frequencies. The surfaces of the filament 1 is carbonated to improve thermoelectron emission efficiency of the filament 1. Therefore, the filament 1 is brittle. When one of the center and side leads 4 and 5 starts vibration, the filament 1 cannot follow the deformation of the vibrating lead 4 or 5 and is thus disconnected. In order to prevent this, the spacer 8 serves to cancel vibration behaviors of the center and side leads 4 and 5. However, when an actual resonance state is observed, the center lead 4 has a larger amplitude since it has a lower resonance frequency than that of the side lead 5 by a value corresponding to a difference between lengths thereof.
As shown in FIG. 1B, the spacer 8 provides sufficient antiresonance properties along the direction A-A'. However, since the center and side leads 4 and 5 are simply fitted in the spacer 8, the spacer cannot provide sufficient antiresonance properties along the direction B-B'. When one lead starts vibration, the spacer starts pivoting about the other lead along the rotational direction indicated by arrow C-C'.
In order to solve the above problem, a conventional means is provided wherein an expensive material such as molybdenum is used to increase the diameter of the lead. Another conventional means is also provided wherein metallized films are formed in lead through holes and the leads 4 and 5 are fixed by an expensive silver-copper brazing material therein. All these conventional means result in high cost, resulting in inconvenience.